#1148: Bad warnings about unused imports that are in fact needed
-------------------------+--------------------------------------------------
Reporter: brianh | Owner:
Type: bug | Status: new
Priority: lowest | Milestone:
Component: Compiler | Version: 6.6
Severity: trivial | Keywords: warning unused import
Difficulty: Unknown | Testcase:
Architecture: x86 | Os: Windows
-------------------------+--------------------------------------------------
If I compile the following code with the `-W` flag I get a warning about
the import of `Data.Ix` that shouldn't be generated, since this import is
actually needed:
{{{
{-# OPTIONS_GHC -fglasgow-exts #-}
module ArrayBoundedU
( T
, create
, at
) where
import Data.Ix
import qualified Data.Array.Unboxed as Array
import Data.Array.Base (unsafeAt)
newtype T i e = T (Array.UArray i e)
create :: (Ix i, Bounded i, Array.IArray Array.UArray e) => [(i,e)] -> T i
e
create ies = T (Array.array (minBound, maxBound) ies)
at :: (Ix i, Bounded i, Array.IArray Array.UArray e) => T i e -> i -> e
at (T a) i = unsafeAt a (index (minBound, maxBound) i)
}}}
with a sample `Main`
{{{
module Main where
import Data.Ix
import ArrayBoundedU
data I = One | Two | Three
deriving (Eq, Ord, Ix, Bounded)
main = do
let
a = create [(One, 10::Int), (Two, 20), (Three, 30)]
putStrLn (show (at a Two))
}}}
built using ghc6.6 with:
{{{
ghc --make Main.hs -W
}}}
I get the warning:
{{{
ArrayBoundedU.hs:8:0:
Warning: Module `Data.Ix' is imported, but nothing from it is used,
except perhaps instances visible in `Data.Ix'
To suppress this warning, use: import Data.Ix()
}}}
However if I do this then the code doesn't compile at all, since the
import '''is''' needed.
This is just a very trivial little thing but it prevents me from being
able to get my code to compile with no warnings (unless I switched the
warning off altogether but then something else which really was unused
would go undetected).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1148>
GHC <http://www.haskell.org/ghc/>
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